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Wednesday, 31 October 2012

RO Machine - Part 3

Part 3 of the build started out on a high.

I checked the seemingly bodged idea of using putty to seal up the sensor mounts…

01-RO Machine.27

Works perfect! Completely water tight!

So i set about wiring up the sensor and power cables to the PS-202…

02-HM Digital PS-202 TDS Controller.52

Attached the sensors into a bit of a test rig…

03-HM Digital PS-202 TDS Controller.33

And set about running tap water through them to see what happened. With the PS-202 powered up, sure enough there were TDS readings on the screens. Happily both showing the same thing…

04-HM Digital PS-202 TDS Controller.21

However, i was slightly confused, as my other instruments i use, namely an inline TDS meter on the RO unit and a TDS Pen, both show the tap water TDS to be around 270. So this appears to be reading a bit low in comparison.

This has sent me into a bit of a whirl of confusion today.

For a reference check i ran some straight pre made RO through it…

06-RO Machine.48

Both scales showed zero…

05-HM Digital PS-202 TDS Controller.37

I was happy enough with this so far…

Next i combined it into the RO units pipework…

07-RO Machine.46

I then proceeded to prepare the unit in the manner i have been doing for the last several months. Running off the membranes down to around 15TDS before switching the flow into the DI chamber, followed by running that off until it hits 0TDS and then start collecting in the storage tank. Essentially this is what the automated controller will be taking care of in future.

Some of these next images are a little shaky as i had to quickly switch back and forth to capture the numbers passing by…

To start with the “IN” channels on both TDS meters were showing about the same, with as much as i could keep up with the photographs…

08-RO Machine.02

That says 134ppm

09-HM Digital PS-202 TDS Controller.04

Once 15TDS was hit i switched it to transfer to the DI pod and allowed that to run off to zero. Once it was in full production, i checked the values out.

The inline TDS monitors “IN” channel showed 5, which is the water coming out of the membranes, this is normal, especially with the boost pump running too…

11-RO Machine.19

The “OUT” channel on the monitor, which shows the water leaving the DI pod, was zero, as required…

12-RO Machine.55

Interestingly on the PS-202 both the IN and OUT channels mounted directly alongside the respective sensors of the existing monitor both showed zero?

10-HM Digital PS-202 TDS Controller.15

I do expect to see a few on the “IN” channel, or the membrane output (depending how you look at it).

At first i wondered if the PS-202 was perhaps more accurate and calibrated better, and indeed the manual says the unit is factory calibrate to 342ppm using NaCl… also known as salt. It also has full temperature compensation and the water running through the sensors was quite cool at around 10degrees. But that's what the temp comp should be taking care of and adjusting the value as if it were 25degrees, this so led me to believe that perhaps the other monitor was perhaps over reading due to not having temperature compensation?

I wont got too deep into the science here, but you can read an excellent article on the whole thing by Randy Holmes-Farley on the following link…

What is TDS - By Randy Holmes-Farley

So I'm currently stuck on what to believe? I would hope the more advanced and more expensive PS-202 would be the bottom line answer… but I'm not so sure at the moment.

One thing i did attempt is a homemade calibration solution using table salt.

After a quick hunt round the internet it would appear 1mg/l of NaCl will give a 1ppm reading. To work to the 342ppm calibration that means i would need 342mg/l. Sadly i don't have a set of scales capable of working down to milligrams!

So i made a solution of 1g with 2litres of RO water, which should give me 500mg/l or 500ppm.

15-RO Machine.53

I checked it first with my handheld tester, the result was promising…

16-RO Machine.46-001

Ok a little high, but this may be down to the measuring of the salt and the water.

So i checked it on the PS-202, in both a static (by filling the sensor fittings) and flowing (by pumping it to and from a jug) and got a reading of about 100ppm less…

17-HM Digital PS-202 TDS Controller.18

Even more worrying, i checked the other TDS monitor and got about 900ppm! With both standing and flowing being more or less the same…

18-RO Machine.23

This baffled me completely! There must be something very wrong to the solution or the method i am trying. But each test device repeated its results every time i redipped their respective sensors…

This confused and annoyed me, as the pen and the inline monitor both more or less the same numbers for tap water, but are way off with the artificial solution? Even if it wasn't mixed up to any specific quantity id assume a reasonable number match with 10% when looking at each one?

So I've ended up a little stuck here for now while i think it over. It seems wise to purchase a proper calibration solution from a reputable source and take it from there.

I did pick up and start thinking about other parts of the system like the solenoid control, but the bother from the TDS results caused me to loose interest a little!

So i put it down for the day and shall return later with a fresh mind, before i start doing anything idiotic!

Just to finish of, a simple project box will be used to house the solenoids and their pipework, with some external connections to allow the box to be connected into the RO unit…

13-RO Machine.40

14-RO Machine.58

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